There are various types of imaging devices which may be used to obtain images of an eye, for example ophthalmoscopes and fundus cameras. Specifically, such devices take measurements of the retina of the eye, which is a three dimensional structure approximated by a sphere. The output of the imaging devices is a two dimensional image of the three dimensional retina, as two dimensional images have a number of advantages, not least that they are able to be represented on a computer screen and output as photographs etc. The imaging devices inspect, e.g. scan, the three dimensional retina and produce two dimensional images of the retina. In inspecting the three dimensional retina, the devices introduce distortion, which distortion is determined by the optical and mechanical properties of the devices. The distortion compromises one or more properties of the two dimensional images of the retina, for example distance, area and angle preservation. If geometrical measurements of a retina are required, such as the distance between two structures of the retina, and these are made using a two dimensional image produced by such imaging devices, the results will be distorted and inaccurate.